

        var changingBg = null;
        function changeBg(){
            var topVal = $('body>.bottomBg .box .son .sawtooth').height(); 
            if(document.documentElement.scrollTop || document.body.scrollTop > (document.documentElement.scrollHeight || document.body.scrollHeight) * 0.15){
                var scrollHeight =  document.documentElement.scrollHeight || document.body.scrollHeight;
                var height = scrollHeight - window.innerHeight - (document.documentElement.scrollHeight || document.body.scrollHeight) * 0.15;
                var val = (document.documentElement.scrollTop || document.body.scrollTop) - (document.documentElement.scrollHeight || document.body.scrollHeight) * 0.15;
                var box_h = $('.bottomBg .box')[0].offsetHeight;
                var top = box_h - parseInt(val / height * (box_h + topVal));
                $('.bottomBg .son').css('top',top + 'px');
            }
        }
       
        $(document).ready(function(){
            changeBg();
            $(window).scroll(function(){
                clearInterval(changingBg);
                changingBg = setInterval(changeBg,10);
            });
        })
        /*document.addEventListener('touchmove', function () {
            clearInterval(changingBg);
            changingBg = setInterval(changeBg,10);
         },false)*/